Antagonism of prostaglandin-induced cyclic AMP accumulation in the rat anterior pituitary in vitro by somatostatin analogues

Mechanisms of somatostatin action in pituitary cells

We have examined the mechanisms by which S-14 inhibits pituitary hormone secretion in a homogeneous cell population: the clonal GH4C1 cell line. The S-14 receptor in GH4C1 cells is coupled to Ni, a guanine nucleotide binding protein which mediates S-14-induced inhibition of VIP-stimulated adenylate cyclase activity, cyclic AMP production and hormone secretion. In addition, a functional Ni is required for S-14 to inhibit basal hormone secretion, an action which appears to be independent of cyclic AMP concentrations. Accumulating evidence indicates that the mechanism of S-14 action in somatotrophs is similar to that in GH4C1 cells. Although S-14 consistently inhibits basal GH secretion, its effects on basal cyclic AMP levels in normal pituitary cells are variable and often not significant (10-14). In contrast, S-14 inhibits prostaglandin and growth hormone releasing factor (GRF) stimulated cyclic AMP accumulation and GH release in parallel. Furthermore, S-14 partially blocks prostaglandin and GRF stimulation of adenylate cyclase activity in rat anterior pituitary membranes. Finally, pretreatment of primary cultures of rat pituitary cells with IAP antagonizes S-14 inhibition of both basal and GRF-stimulated GH release.

Somatostatin lowers the cytosolic free Ca2+ concentration in clonal rat pituitary cells (GH3 cells)

Changes in the cytosolic free Ca2+ concentration, [Ca2+]i, have been proposed to mediate the regulation of the secretion of pituitary hormones by hypothalamic peptides. Using an intracellularly trapped fluorescent Ca2+ probe, quin2, [Ca2+]i was monitored in GH3 cells. Somatostatin lowers [Ca2+]i in a dose dependent manner from a prestimulatory level of 120 +/- 4 nM (SEM, n = 13) to 78 +/- 9 nM (n = 5) at 10(-7)M; the effect is half maximal at 2 X 10(-9) M somatostatin. The decrease in [Ca2+]i occurs rapidly after somatostatin addition and a lowered steady state [Ca2+]i is maintained for several minutes. Somatostatin does not inhibit the rapid rise in [Ca2+]i elicited by thyrotropin releasing hormone (TRH) and can still cause a decrease in [Ca2+]i in the presence of TRH (10(-7)M). Concomitantly with its action on [Ca2+]i somatostatin causes hyperpolarization of GH3 cells assessed with the fluorescent probe bis-oxonol. The lowering of [Ca2+]i by somatostatin is however not only due to reduced Ca2+ influx through voltage dependent Ca2+ channels, since it persists in the presence of the channel blocker verapamil. These results suggest that somatostatin may exert its inhibitory action on pituitary hormone secretion by decreasing [Ca2+]i.

Effects of vasoactive intestinal polypeptide, monoamines, prostaglandins, and 2-chloroadenosine on adenylate cyclase in rat cerebral microvessels

Adenylate cyclase in microvessels isolated from rat cerebral cortex was stimulated by guanine nucleotides, catecholamines, prostaglandin E1, prostaglandin E2, and 2-chloroadenosine. Catecholamine stimulation was mediated by interaction with beta-adrenergic receptors. The order of relative potency was: isoproterenol greater than epinephrine greater than norepinephrine. Activation of microvessel adenylate cyclase by prostaglandins E1 and E2 as well as by 2-chloroadenosine was dose related. Twenty-two peptides were tested for possible effects on the microvessel adenylate cyclase. Only vasoactive intestinal polypeptide (VIP) was stimulatory. No inhibitory action was observed. Activation by VIP required guanosine triphosphate and was dose dependent from 10 nM to 1 microM (ED50 = 0.1 microM). At 30 degrees C, stimulation of adenylate cyclase by the peptide increased linearly with time for up to 15 min. The effect of VIP was not inhibited by phentolamine or propranolol, suggesting that its action was not elicited by interaction with alpha- or beta-adrenergic receptors. Activation achieved by VIP and isoproterenol, prostaglandin E1, or 2-chloroadenosine was the sum of the individual stimulations, suggesting that receptors for VIP were distinct from those for isoproterenol, prostaglandin E1, and 2-chloroadenosine.

Role of cyclic nucleotides in the inhibition of growth-hormone secretion by somatostatin

The effects of somatostatin on GH secretion, cyclic AMP and cyclic GMp concentrations in dispersed bovine anterior pituitary cells were studied following activation of adenylate cyclase with cholera toxin and inhibition of phosphodiesterase with isobutylmethylxanthine (IBMX). Cholera toxin (10(-5)M) increased intracellular cyclic AMP concentration 10-fold and cyclic GMP concentration 3-fold relative to control, and stimulated the secretion of GH. IBMX (10(-4) M) als increased intracellular concentrations of both cyclic AMP and cyclic GMP and the secretion of GH and potentiated the actions of cholera toxin particularly in raising intracellular cyclic AMP concentrations which were elevated 40-fold in the presence of cholera toxin and IBMX. Somatostatin (5 X 10(-7) M) completely prevented GH secretion elicited by cholera toxin and/or IBMX. Somatostatin was without effect on control cyclic AMP and cyclic GMp concentrations and on the increases in both cyclic AMP and cyclic GMP caused by cholera toxin and by IBMX alone, or in combination. The data suggest that bovine GH secretion is increased when concentrations of either or both cyclic nucleotides are elevated within the cells, although incubation of cells with extracellular concentrations of cyclic AMP and cyclic GMP derivatives up to 2 x 10(-3) M caused only small changes in GH release. We suggest that somatostatin inhibits cholera toxin-induced bovine GH secretion by preventing activation of the secretory process by either cyclic AMP or cyclic GMP.

Inhibition of prostaglandin-induced cyclic AMP accumulation in the rat anterior pituitary by alrestatin

Alrestatin at 25-1 X 10(-4) M inhibited the accumulation of cyclic AMP induced by prostaglandin E2, but not theophylline, in the rat anterior pituitary in vitro. Somatostatin, at lower concentrations, inhibited both; maximal inhibition of the prostaglandin effect was greater with alrestatin. As cyclic AMP is considered to be a mediator in induced-hormonal release, it appears from the present findings that alrestatin may be of potential use in altering hormonal release.

Inhibition in the rat of gastric acid secretion and cyclic AMP analogs accumulation in vitro by somatostatin

Various somatostatin (S) analogs exhibited similar degree and similar, or shorter, duration of inhibition of basal gastric acid secretion as S in the unanesthetized rat and similar, or less, inhibition of the cyclic AMP accumulation induced by prostaglandin E2 in the rat anterior pituitary in vitro. With the analogs examined, the gastrointestinal and pituitary receptors appear to exhibit generally similar recognition specificity with the differences within the gastro-intestinal activities reflecting duration of availability rather than receptor affinity.